It is well known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor is able to generate electron-hole pairs in the semiconductor. Because of the potential difference which exists at a p-n junction, electrons and holes move across the junction in opposite directions, creating a flow of electric current which can deliver power to an attached load.
Although most solar cells fabricated to date utilize silicon as the semiconductor material, cells have been prepared from other materials, such as cadmium sulfide and gallium arsenide. Silicon is preferred for several reasons: (1) it is a plentiful material and (2) the energy band gap of approximately 1.1 electron volts, responds quite favorably to electromagnetic energy having a wavelength in the visible and ultraviolet regions of the solar spectrum. The performance of silicon solar cells is particularly related to the degree of crystalline perfection of the semiconductor, since the flow of electrons is impaired by grain boundaries in the crystal. Thus, monocrystalline silicon exhibits the best performance, and this performance decreases upon addition of grain boundaries in a polycrystalline device. Thus, the smaller the grains, the greater the grain boundary density which impedes the current flow. In addition, the performance of silicon cells is affected by other defects such as increased level of impurities in the material and line dislocations.
Numerous methods have been developed to produce thin silicon solar cells. Unfortunately, economical production techniques have not been devised to prepare high quality silicon for such solar cells. Significant decreases in cost have been achieved to reduce the price of silicon solar cells, but more substantial cost reductions are required to make photovoltaics economically competitive with alternative sources of energy for uses other than remote applications, such as powering earth satellites.
Currently, silicon solar cells are manufactured in a multi-step process. Polycrystalline silicon is prepared by reducing trichlorosilane with hydrogen. Ingots of monocrystalline silicon are then grown from the polycrystalline starting material, and silicon wafers are prepared by cutting the resulting single crystal ingot to a thickness of at least 0.25 mm. Because of damage caused to the thin silicon wafer by cutting with a diamond saw, the silicon must subsequently be polished or etched to prepare the material for solar cell use. A dopant is then diffused into the silicon to form a shallow p-n junction, ohmic contacts are applied to the rear surface, grid contacts are attached to the diffused surface, anti-reflection and protective coatings are applied to the diffused surface, and the cell is mounted into position. Although it is apparent that this intricate procedure will necessarily result in high costs, many of the steps, particularly the latter ones, are essential. The crystal growth and sawing are the most expensive steps. Thus, efforts to reduce the cost of silicon solar cells have concentrated on less complex means to produce the thin silicon wafers prior to doping and final processing.
Three types of processes have been utilized to date to produce silicon crystals for fabrication into solar cells. In the Czochralski crystal growth method a seed crystal of silicon is immersed in a melt of formerly polycrystalline silicon contained in a quartz crucible. The seed crystal is slowly withdrawn, extracting with it a new, single crystal of silicon. As indicated in Table I, which compares various processes for producing silicon solar cells, crystalline growth by this process is slow and a high level of skill is required to implement this technique at the manufacturing level.
TABLE I __________________________________________________________________________ CZOCHRALSKI DENDRITIC RIBBON- CRYSTAL FLOAT WEB RIBBON PRESENT PROCESS GROWTH ZONE CASTING EFG GROWTH FLOAT ZONE INVENTION __________________________________________________________________________ Typical 2 4 .about.6 .rarw. .about.30 .fwdarw.* Growth Rate (mm/min) Max. Width 150 100 .about.100 100 50 50 50 Achieved (mm) Throughput 80 70 .about.100 1-2 .fwdarw. .fwdarw. .fwdarw.* Achieved (gms/min) Crystal Single Single Multi- Multi- .about.Single Multi- Large multi- Structure grained grained grained grained Technology- High Ex. High Low High High Ex. High Low skill Sawing Yes Yes Yes No No No No Required Other Possible Low yields/ Small Delicate Delicate Delicate Contamina- Process Contamina- difficult Grain thermal temp. thermal tion from Problems tion by process to Size control/ monitor- control. filaments. crucible. initiate/ die contam- ing req'd, delicate ination wide growth thermal is difficult. control. Solar Cell 16 16 11 11 15 9 13.8 Efficiency (%) __________________________________________________________________________ *Growth rate and throughput are expected to be higher by a factor of approximately 20 in the horizontal growth mode.
A second method of forming silicon bars is the "float-zone" method. A rod of polycrystalline silicon is moved slowly downward through a heater to form a localized zone of molten silicon through the cross-section of the rod within the heater. A silicon seed crystal in contact with the liquid is slowly pulled downward to extract a single crystal of silicon from the melt zone. More of the polycrystalline rod is lowered into the melt zone as single crystal material is withdrawn from the bottom. Again, the level of skill required to move the polycrystalline rod and the seed crystal and to maintain a proper float zone is extremely high. As indicated in Table I, the rate of crystalline growth is quite low, approximately 4 mm/min.
In the third alternative, silicon bars may be formed by casting liquid polycrystalline silicon in a crucible, and then removing the solidified silicon from the crucible. In a variation of this casting process, liquid silicon is poured onto a rotating drum from which the solidified silicon ribbon is removed. Although the skill required for such a process is relatively low and silicon bar growth is faster than the two previously mentioned processes, the crystal end product is not as good as the first two methods discussed. The cast silicon product is characterized by small grain size, and solar cell efficiency is less than 70% of that demonstrated by the monocrystalline silicon wafers produced by the two methods just discussed.
To remove the necessity for sawing and polishing the silicon bars to produce semiconductor wafers adequate for solar cell use, attempts have been made to produce silicon in thin layers. Although silicon melts at 1415.degree. C., it is malleable in a narrow temperature range in which it has a tendency to collect impurities easily. Thus, attempts to use conventional metallurgical techniques, such as rolling, to form thin silicon sheets have been unsuccessful and more sophisticated methods have been required.
One of these processes, the "ribbon-ribbon float zone" technique, is an adaptation of the float zone technique described previously with the substitution of polycrystalline silicon ribbon for the feedstock. While the resulting crystalline silicon film is easier to fashion into finished cells than the rod material, the use of the film increases the difficulty of process control in the delicate melt zone region. In addition, the laser heating techniques usually employed in this process are quite inefficient, and crystal quality is poor.
Another method utilized to make silicon ribbon is the edge defined, film-fed growth or "EFG" process which employs a silicon melt in a crucible much like the Czochralski process. In the EFG process, however, a die is placed in the melt so that part of the die extends above the liquid surface. Capillary forces cause the liquid silicon to rise through the internally defined spaces of the die, and crystalline silicon is slowly withdrawn by a seed crystal pulled from the top of the die. As illustrated in Table I, the EFG process achieves significantly higher pulling rates than the Czochralski method, but the product is a multi-grained crystalline structure which substantially reduces solar cell efficiency. In addition, delicate temperature control is required at the top of the die. Too much heat will increase liquidity and cause separation of the silicon melt from the seed crystal; too little heat will cause the silicon to freeze in the die. For silicon capillary rise, the die material should be wettable by silicon which usually means the material is also soluble in the silicon. For example, graphite, a typical die material, dissolves in and contaminates silicon.
Finally, attempts have been made to produce thin films of silicon by "dendritic web growth". In this technique a supercooled melt of silicon is prepared, i.e., the temperature at lower layers of the melt are cooler than the surface of the melt. A single dendritic crystal is inserted in the melt and slowly withdrawn to form a web of crystalline material. This technology is set forth by Barrett, D. L., Myers, E. H. Hamilton, D. R., Bennett, A. I.: J. Electrochem. Soc. 118, 952 (1971). This technology is extremely difficult to execute, particularly in maintaining proper control over the supercooled silicon melt.
Thus, it is an object of this invention to produce thin, crystalline semiconductor sheets suitable for use in photovoltaic devices while avoiding the difficulties of the prior art.
It is also an object of the present invention to produce high quality crystalline silicon sheets by a process employing relatively unsophisticated semiconductor technology which is capable of being mechanized for continuous operation.
It is another object of the invention to provide an apparatus which is of simple construction and is able to produce crystalline semiconductor sheets suitable for use in solar cells without excessive finishing steps.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.